The present invention relates to a system (and method) for recovery of degraded images, and relates particularly to, a system for recovery of degraded or blurred digital images captured through atmospheric turbulence, or other obscurations, such as fog, smoke, snow, rain, or underwater fluctuations, using a reference point source or object in each captured image. The invention is especially suitable for improving the quality of images in poor weather conditions.
Typically, atmospheric turbulence, such as caused by the weather, can cause degradation in the quality of images taken by telescopes and cameras. Several approaches have been used to deal with the problem of imaging through turbulence. In the area of astronomy, adaptive optics correct each frame by sensing the wavefront distortion induced by the turbulence and providing a servo-controlled phase screen, often referred to as a rubber-mirror. The use of such adaptive optics is described, for example, in H. W. Babcock, Publ. Astron. Soc. Pac. 65 (1953) 229, and R. K. Tyson, PRINCIPALS OF ADAPTIVE OPTICS, 1991. Imaging systems using adaptive optics to correct for atmospheric turbulence are complex and expensive.
Another approach for imaging through turbulence is speckle interferometry. Using a large sequence of frames taken at short exposure times through a telescope, the effect of atmospheric turbulence are reduced by processing the frames through Fourier transforms. Speckle interferometry is described, for example, in A. Labeyrie, Astron. Astrophys. 6 (1970) 85, K. T. Knox et al., xe2x80x9cRecovery of Images from Atmospherically Degraded Short-Exposure Photographs,xe2x80x9d Astrophys. J. 1993 (1974) L45, and F. Roddier, The effects of atmospheric turbulences in optical astronomy, in: Process in Optics XIX, 1981.
More recently, digital processing techniques are being used to correct images for atmospheric turbulence. In an article by B. R. Frieden, xe2x80x9cAn exact, linear solution to the problem of imaging through turbulence,xe2x80x9d Opt. Comm. 150 (1998) 15, a sequence of two short-exposure intensity images is taken without any reference point sources, and the images are Fourier transformed and divided by linear equations based on two random point spread functions. The result is inverse filtered to provide an image of an object. One problem with this method is that the point spread function associated with the turbulence is not known in an image due to the lack of any reference, which can cause difficulty in recovering an image taken through turbulence. Accordingly, it would be desirable to more accurately recovery degraded images by using a reference point source or object in each image captured, which can be used to characterize the atmospheric turbulence as a point spread function.
In the area of digital image coding and compression, digital images may be processed as described in U.S. Pat. No. 5,453,844 using a random blurring function consisting of numerous, irregularly spaced delta functions. The patent provides for secure transmission of coded images.
It is the principal object of the present invention to provide an improved system for recovery of degraded images captured through atmospheric turbulence or other turbulent media, which is less expensive than the prior art adaptive optical systems.
Another object of the present invention is to provide an improved system for recovery of degraded images by imaging a reference point source in each of the degraded images.
A further object of the present invention is to provide an improved system for recovery of degraded images in which a reference object in the degraded image is used to recover the image of an unknown object.
A still further object of the present invention is to provide for an improved system for recovery of degraded images in which images can be recovered in approximately real-time.
Briefly described, the system embodying the present invention includes an imager for capturing through atmospheric turbulence, or other turbulent media, a degraded image of a scene having at least one object, and an image of a point source associated with the object. The point source may be provided by a laser which produces a beam reflecting from a reflector attached to, or in proximity of, an object in the scene, or the point source may be provided by a laser attached to, or in proximity of, an object in the scene. The imager converts the degraded image into first image data signals representing the degraded image, and converts the image of the point source into second image data signals representing a point spread function. A computer of the system receives the first and second image data signals and produces third image data signals representing a recovered image of the object of the degraded image in accordance with the first and second image data signals.
The imager represents an image capturing unit having optics for capturing light having both the degraded image and the image of the point source, and separating the light representing the image of the point source from the light representing the degraded image. A CCD in the unit detects the light representing the point source image to provide the second image data signals. A digital camera having one or more CCD""s detects the light representing the degraded image to convert the light into the first image data signals. An output device coupled to the computer can output the image data signals representing a recovered image to display or print the recovered image.
In another embodiment of the present invention, the imager represents a digital camera which captures a degraded image through atmospheric turbulence of a scene which includes a degraded image of a known reference object and an unknown object. The computer after receiving the first image data signals of the degraded image identifies in the first image data signals such image data signals representing the reference object. The computer produces image data signals representing a recovered image of the scene with the unknown object in accordance with the first image data signals, the image data signals representing the reference object in the first image data signals, and image data signals representing an undegraded image of the reference object. The image data signals representing the undegraded image of the reference object were captured previously, when no or minimal atmospheric turbulence was present, and stored in memory accessible by the computer. An output device coupled to the computer can output the image data signals representing a recovered image to display or print the recovered image in which the unknown object is now viewable.